1. Field of the Invention
This invention relates to a key telephone system and in particular to a signal priority arrangement for "hands-free" communication systems.
2. Description of Prior Art
Prior art key telephone system station signal arrangements have used ringers, buzzers, or lighted lamps to indicate the arrival of an incoming call to a party engaged in an existing call. To place an existing call on hold, the called party must operate the hold button on the key telephone set and only then can the incoming call be answered by depressing the appropriate pick-up button.
A prior art call priority arrangement is the executive override feature wherein certain stations assigned a priority class of service can bridge onto an existing call and request that the existing call be terminated. In this fashion, the executive can obtain an idle line based on the class of service of his key telephone set, thus establishing a rudimentary call priority arrangement.
An improvement over these two arrangements is shown by the teaching of the Shinoi et al patent, U.S. Pat. No. 3,904,834, issued Sep. 9, 1975 wherein station signaling is combined with call priority. The Shinoi et al patent shows a key telephone system in which stations are equipped with speakerphone facilities for use in both intercom and central office calls. When an incoming intercom call is directed to a key telephone station presently engaged in a speakerphone central office call, the speakerphone equipment in the called station is switched into the receive mode and both incoming intercom and central office call signals are simultaneously broadcast through the speakerphone loudspeaker. No warning tone is given to the parties engaged in the existing call and, as in other prior art systems, the user at the called key telephone station must operate a switch to place the existing call on hold before the incoming call can be answered.
In prior art station signaling arrangements, the called party is required to manipulate the station set to answer an incoming call. This is true of both loudspeaking intercom systems as well as standard key telephone systems. Additionally, prior art station signaling arrangements have either no call priority scheme or at best only a limited call priority scheme. The Shinoi patent teaches a limited call priority scheme wherein incoming intercom calls can override an existing central office call but as to the other types of calls in the key telephone system, the Shinoi arrangement has no call priority scheme. The aforementioned executive override arrangement provides executive phones with priority over existing intercom calls but otherwise the associated telephone system has no call priorities.
In key telephone systems, it is obviously desirable to provide a complete hierarchy of call priorities so that certain types of calls can preempt others. In loudspeaking key telephone systems, it is desirable to provide facilities to enable the called party to answer an incoming preempting call "hands-free" without having to manipulate the station set.
In view of the foregoing, an object of this invention is to provide a complete hierarchy of call priorities in a loudspeaking intercom system such that the called party is able to answer incoming preempting calls "hands-free" without having to manipulate the station set.
It is a further object of this invention to provide a tone indication to the parties engaged in the existing call to indicate the reason for the call interruption.